There are a variety of different types of circuits which use resonant tunneling devices, such as resonant tunneling diodes. A resonant tunneling diode (RTD) inherently includes an intrinsic capacitance, which is generally negligible during operation at frequencies below a transition frequency of approximately 100 GHz. In particular, below approximately 100 GHz, the intrinsic capacitance has a relatively high reactive impedance in comparison to the intrinsic resistance of the RTD, and thus the resistance dominates and the intrinsic capacitance is effectively negligible. However, the reactance of the intrinsic capacitance progressively decreases as the frequency progressively increases. Consequently, above the transition frequency, the impedance of the RTD is dominated by the intrinsic capacitance, rather than the intrinsic resistance.
Since it is the resistance characteristic of an RTD which is of interest to circuit designers, the intrinsic capacitance has the effect of rendering the RTD substantially unsuitable for use at frequencies above its transition frequency. Consequently, circuit designers avoid using an RTD in any circuit that operates at a frequency higher than the transition frequency of that RTD. However, since RTDs are the fastest known semiconductor switches that have intrinsic gain, there are a variety of high-frequency circuits in which the use of RTDs would be highly desirable, except for the fact that these circuits have operating frequencies well above the transition frequency of the typical RTD.